1. Field of the Invention
This invention relates to a fixing method of developing powder to be used for the electrophotographic method, electrostatic recording method, magnetic recording method, and so forth.
2. Description of the Prior Art
For the electrophotographic method, there have so far been known various types such as those in U.S. Pat. No. 2,297,691, British Pat. No. 1,165,406, British Pat. No. 1,165,405, and so forth. These methods are to obtain, by various means, reproduced images through the process steps of forming an electrical latent image on a photosensitive member constructed with a photoconductive material, developing the latent image using a developing toner, then transferring the toner image onto an image transfer material such as paper, depending on necessity, and finally fixing the thus transferred image by heat, pressure, or solvent vapor. When the step of transferring the toner image onto the image transfer material is include in the whole process steps, there is further added a step of removing residual toner.
The method of visualizing the electrical latent image with use of a toner has been known, for example, from U.S. Pat. No. 2,874,063 disclosing the magnetic brush method, U.S. Pat. No. 2,618,552 disclosing the cascade developing method, U.S. Pat. No. 2,221,776 disclosing the powder clouding method, and U.S. Pat. No. 3,909,258 disclosing a method of using an electrically conductive magnetic toner, and others.
For the toner suitable for these developing methods, there has so far been used fine powder of natural or synthetic resin material, in which dyestuff and pigment are dispersed. For example, pulverized powder particles of 1 to 30 microns or so in diameter prepared by dispersing a coloring matter in a binder resin such as polystyrene are used as the toner. For the magnetic toner, there has been used magnetite, and the like containing therein magnetic particles. In the case of the system which utilizes the so-called two-component developer, the toner is used in mixture with carrier particles such as glass beads, iron powder, etc.
These toners are required to have various physical and chemical properties, but most of the known toners have some defects to be mentioned as follows. That is, most of the toners which readily melt by heating tend to be caked or coagulated during storage or within the reproduction apparatus. Most of the other toners deteriorate their triboelectric property and fluidity due to temperature changes in the surroundings. Most of still other toners cause change in the image density or increase in the background density due to mutual deterioration among the toner, the carrier particles and the photosensitive member occurring from collision among the toner particles and the carrier particles as well as contact between these particles and the surface of the photosensitive plate member as the result of repetitive development owing to continuous use of the reproduction apparatus. Further, most of yet other toners causes increase in the background density when the density of the reproduced image is to be augmented by increasing the adhering quantity of the toner to the surface of photosensitive plate member, thereby causing the so-called fogging phenomenon.
Of these various undesirable phenomena as mentioned above, there is such one that occurs due to brittleness of the toner particles. Being brittle, the toner particles are readily pulverized by mechanical fore, which appears preferable from the standpoint of productivity of the toner. On the other hand, however, such toner particles are liable to be readily crushed even by a load applied to them within the developing device and become pulverized to bring about undesirable phenomena such as contamination of the carrier particles and the developing sleeve, or fogging phenomenon to the reproduced image due to the toner particles per se becoming unable to be perfectly controlled in their charge. Such brittleness in the toner particles greatly affects the life of the developing agent.
With a view to avoiding such deteriorating phenomena, there has been contemplated use of various polymerized substances having high molecular weights. In consideration, however, of the image fixing under heat which is usually done at the ultimate step of the reproduction, since the fixing temperature should be elevated at the time of the image, fixing with such polymer substance, necessitating much more heat quantity, it is not so preferable from the standpoint of the energy saving. It has also been proposed to add a small amount of a plasticizer in the toner to eliminate such deteriorating phenomenon. This proposal is again problematical in that it impairs fluidity of the toner and also contaminates the carrier particles, etc., hence it is not always successful.
Conversely, when the toner is too hard, mechanical crushing is not feasible to make it difficult to practically manufacture the toner.
For various reasons as mentioned in the foregoing, there has so far been used, as the binder resin for the toner, polystyrene or copolymers of styrene and butyl methacrylate having a relatively low molecular weight of a few to several thousands and having an appropriate hardness.
Of recent, however, strong demands have been clamoured from among various circles of users as to improvement in operational reliability of the reproduction apparatus. On the part of the manufacturers of the reproduction apparatus, they are now endeavoring in development and manufacture of such reproduction apparatus having a longer service life from the standpoint of maintenance. Under such situations, when the various characteristics of the toner are reviewed, it has been discovered that polystyrene or copolymers of styrene and butyl methacrylate of relatively low molecular weights as mentioned above are not satisfactory in their hardness as the binder resin for the toner, and that materials having higher hardness are required.
The image fixing method which has been most generally diffused in the reproduction apparatus in recent years is the fuser roller fixing method. In most of the fuser roller image fixing device incorporated in the reproduction apparatus which is now commercialized, it is the usual practice to apply oil on the surface of the roller. However, this oil application causes disagreeable feeling to an operator due to vaporization of the oil, stain of the sheet due to the oil, complexity in the construction of the fixing device which is liable to mechanical trouble not infrequently, and increase in the manufacturing cost, and various other undesirable problems. Accordingly, it has been expected that the fuser roller image fixer with no oil applied on the roller or, if applied, with very small quantity of application would emerge. Such improvement cannot be realized without improvement in the toner itself. The very point of difficulty in adopting the fuser roller image fixing device without oil being applied on the roller is that the toner should have sufficient separability from the image fixing roller to compensate the non-oil-application. Therefore, it is difficult to obtain the toner having anti-offset property and anti-twining property, while maintaining as low a fixing point as possible, and it is more difficult to obtain the toner which is excellent in both image fixing and developing characteristics.
In the conventional toner, the binder resin is such one that it has a molecular weight distribution curve with one molecular peak value, or it has a plurality of peak values in the low molecular weight region, or it is a mixture of entirely different compounds, each having different molecular weight distribution.
Such toner as mentioned above cannot be said to be excellent toners in both image fixing and developing characteristics.
There have so far been proposed various methods for improving the image fixing property of the toner with respect to the fuser roller fixing. For example, U.S. Pat. No. 3,941,898 teaches the toner, in which a cross-linked polymer is used as the binder resin. According to this patented method, remarkable improvement can be seen in respect of the anti-offset property and the anti-twining property, but there still remain various disadvantages such that no satisfactory image fixing property can be obtained by mere cross-linking since the fixing temperature is still low, and that, as the cross-linked polymer is difficult to disperse pigment and is incompatible with other polymers, the toner using such cross-linked polymer as the binder resin is difficult to have satisfactory developing characteristics. Also, German laid-open patent application DOLS 2352604 proposes a toner, in which a low molecular weight polypropylene is blended with a styrene type resin. According to the invention as laid open, it is necessary to include a large amount of low molecular weight polypropylene so as to obtain a satisfactory effect with respect to the anti-offset property. On the other hand, however, coagulating property of the toner increases to disadvantageously deteriorate its developing characteristics.